As is well known, one of the primary roles of a head drum in a VCR is to provide stability to the magnetic tape travelling along a defined path within the VCR without damaging or adversely affecting the tape. In order to fulfill this role, the surface of the head drum which is in contact with the moving tape is designed in such a way that it has macroscopic smoothness and yet, microscopically, is provided on its surface with a plurality of undulations running along the direction normal to the rotational axis thereof in order to reduce the friction therebetween. Inasmuch as another design requirement thereof lies in the protection of the information recorded on the tape, the head drum is usually made of a non-magnetic material .
The head drum has been hitherto made primarily by die casting an aluminum alloy such as Al--Si, Al--Cu or a variation thereof. Above the surface of the head drum made by die casting such an aluminum alloy, there is formed a layer of aluminum oxide (Al.sub.2 O.sub.3) in the thickness of a few nm, said layer being in physical contact and interacting with the lubricating and magnetic layer of the tape.
Since the layer of Al.sub.2 O.sub.3 formed on the head drum surface is of an insufficient thickness, i.e., too thin, the mechanical properties of the Al.sub.2 O.sub.3 layer will be largely influenced by the mechanical properties of the aluminum alloy constituting the aluminum head drum. In other words, the effect of the Al.sub.2 O.sub.3 layer can be virtually ignored and the overall mechanical properties of the aluminum head drum surface can simply be regarded as those of the surface made of the aluminum alloy alone. The mechanical hardness, and hence the resistance to wear, of the aluminum head drum is further affected adversely by the presence of voids or pores therein during the die casting thereof.
During a normal operation of the VCR, since such magnetic powders having a needle-like shape in the magnetic layer as .gamma.-Fe.sub.2 O.sub.3, Fe and the like are mechanically harder than the aluminum alloy as shown in Table 2 hereof, the magnetic powders will, in addition to scratching and wearing down the aluminum head drum surface, become embedded on the aluminum head drum surface. Those embedded magnetic powders, in addition to scratching the tape surface, when accumulated in enough quantities, will form a layer on the surface of the aluminum head drum, which will in turn affect magnetically the information stored on the tape itself, thereby harming the quality of the picture.
Furthermore, since the aluminum head drum is designed by taking into consideration such factors as aerodynamic and adhesion properties between the aluminum head drum surface and the tape in order to provide stability to the traveling tape, even a slight wearing down of the aluminum head drum surface, for example in the neighborhood of 0.3 .mu.m, may hamper the aluminum head drum from fulfilling the above-mentioned primary objective.
Furthermore, wearing down of the tips of the undulations provided on the head drum surface will increase the contact area between the tape and the aluminum head drum, which may further aggravate the wearing down process. If this process is allowed to continue, it will affect the performance of the VCR, causing sudden shaking of picture or stoppage thereof.
One of the methods that can be used in overcoming such a problem is to form a protective layer made of a material which is mechanically harder than the magnetic powders in the magnetic layer of the tape on the surface of the aluminum head drum coming in contact with the tape, as suggested by Shibano et al. in the Japanese Public Disclosure No. 209353/1992, entitled "Magnetic Recording/Reproducing Device" and by Takahashi in the Japanese Public Disclosure No. 126459/1992, entitled "Magnetic Recording Reproducing Drum". According to Shibano et al., it is possible to protect the surface of the aluminum head drum coming in contact with the tape by forming a first protective layer made of titanium(Ti) having a thickness of 0.1-0.3 .mu.m followed by forming a second protective layer made of titanium nitride(TiN) having a thickness of 0.5-0.7 .mu.m. On the other hand, Takahashi indicated that it is possible to solve the problem by forming a protective layer made of diamond-like carbon(DLC) on the surface of the aluminum head drum coming in contact with the tape. However, both approaches are burdened with a number of major shortcomings, one of the major shortcomings being that even though it is possible to protect the surface of the aluminum head drum using the approaches suggested by Shibano et al. and Takahashi, the protective layer made of Ti and TiN or DLC fails miserably in its ability to protect the tape itself. Since the material making up the protective layer is much harder mechanically than the magnetic material on the tape, the protective layer will adversely affect the physical integrity of the tape when it comes in contact with the tape, thereby harming the quality of the picture.